This invention relates to a method of preparing silica glass and, in particular, to a method of preparing silica glass by a sol-gel method.
Several methods for preparing silica glass by sol-gel methods are known. Such methods are disclosed, for example, in Nogami, Nakatani et al, "Journal of Non-Crystalline Solids", Vol. 37, 191 (1980), Rabinovitch et al, "Journal of Non-Crystalline Solids", Vol. 47, 435 (1982) and Toki et al, U.S. patent application Ser. No. 642,606 having foreign counterparts of Great Britian 84/18301, Germany P33 90 375.1, France 84900294 and Netherlands 83.20410.
According to the method of Toki et al, a sol solution of fine particle silica uniformly dispersed in a metal alkoxide hydrolyzed solution having a pH adjusted to between 3 and 6 is used to obtain a porous dry gel. The porous dry gel is sintered and a large silica glass, for example, of 15 cm in width, 15 cm length and 0.5 cm in thickness, which cannot be obtained by the other methods is prepared with good yield. Toki's method is the best of the three methods.
The silica glass obtained by sintering in accordance with Toki's method contains impurities such as metal ions, crystals, foams, and irregularly shaped foreign particles. When the silica glass is to be used for purposes requiring high quality glass, for example, as silica substrates for photo-masks or as preforms of optical fibers, the glass formed by this method is not of sufficiently high quality.
Accordingly, it is desirable to provide a method for preparing silica glass which overcomes the disadvantages found in prior art silica glass processes.